ALBERT

Description: The fact that the deep-ocean benthic δ13C minimum shifted from the North Pacific to the South Atlantic during the Last Glacial Maximum is often interpretted as evidence of a change in deep water circulation, such as the development of deep water ventilation in the North Pacific. This study re-evaluates the implications of changes in benthic δ13C gradients by comparing Pacific Deep Water (PDW) δ13C measurements with the values expected for the null hypothesis that PDW ventilation sources remained unchanged throughout the Late Pleistocene. The δ13C compositions of PDW, Northern Component Water (NCW) and Southern Component Water (SCW) are estimated from regional benthic δ13C stacks of 3–6 sites. Changes in PDW δ13C and PDW-SCW δ13C gradients over the past 800 kyr are found to be well described by a constant mixture of 60% NCW and 40% SCW plus a constant Pacific "age" offset of −0.5‰. Thus, an additional ventilation source for glacial PDW (e.g., in the North Pacific) cannot be inferred solely on the basis of changes in the Pacific-South Atlantic benthic δ13C gradient.

Description: The statistical uncertainties in a 1000 year Northern Hemisphere mean temperature reconstruction obtained from 15 proxy chronologies are examined in detail by analysing the range of estimates obtained from all possible subsets of the proxy collection with up to 6 proxies omitted. The study is motivated in part by the large range of recently published reconstructions in the 15th and 16th centuries. The uncertainty estimates support the conclusions of the 3rd and 4th Intergovernmental Panel on Climate Change (IPCC) assessment reports concerning the likelihood that temperatures at the end of the 20th century were likely (greater than 66% confidence) to have been exceptional. It is also shown that the last ten years to date have been warmer than any decade of the past millennium with 95% confidence.

Description: A new pollen sequence from the Lake Mbalang (7°19' N, 13°44' E, 1110 m a.s.l.) located on the eastern Adamawa plateau, in central Cameroon, is presented in this paper to analyze the Holocene African Humid Period (AHP) termination and related vegetation changes at 7° N in tropical Africa, a region where any data are today available. This sequence, spanning the last 7000 cal. yr BP, shows that the vegetation response to this transitional climatic episode was neither abrupt nor really gradual. Forest degradation in this area is initially registered as early as 6100 cal. yr BP and modern savanna was definitely established at 3000 cal. yr BP and stabilized at 2400 cal. yr BP; but a slight forest regeneration episode is observed between 5200 and 4200 cal. yr BP. Moreover, in this area with modern high rainfall, increasing in the length of the dry season during the AHP termination, from 6100 cal. yr BP onward, has primarily controlled vegetation dynamics and above all the disappearance of a forested environment on the Adamawa plateau. Compared to previous studies undertaken in northern tropical and central Africa, this work clearly shows that the response of vegetation to transitional episodes between climatic extremes such as the AHP termination might be different in timing, mode and amplitude according to the regional climate and hydrology of the study sites, but also according to the stability of vegetation before and during these climatic transitions.

Description: Ice core evidence indicates that even though atmospheric CO2 concentrations did not exceed ~300 ppm at any point during the last 800 000 years, East Antarctica was at least ~3–4 °C warmer than pre-industrial (CO2 ~280 ppm) in each of the last four interglacials. During the previous three interglacials, this anomalous warming was short lived (~3 000 years) and apparently occurred before the completion of Northern Hemisphere deglaciation. Hereafter, we refer to these periods as "Warmer than Present Transients" (WPTs). We here present transient 800 kyr simulations using the intermediate complexity model GENIE-1 which suggest that WPTs could be explained as a consequence of the meltwater-forced slowdown of the Atlantic Meridional Overturning Circulation (AMOC) during glacial terminations. It is well known that a slowed AMOC would increase southern Sea Surface Temperature (SST) through the bipolar seesaw. Observational data supports this hypothesis, suggesting that the AMOC remained weak throughout the terminations preceding WPTs, strengthening rapidly at a time which coincides closely with peak Antarctic temperature. In order to investigate model and boundary condition uncertainty, we additionally present three ensembles of transient GENIE-1 simulations across Termination II (135 000 to 124 000 BP) and three snapshot HadCM3 simulations at 130 000 Before Present (BP). These simulations together reproduce both the timing and magnitude of WPTs, and point to the potential importance of an albedo feedback associated with West Antarctic Ice Sheet (WAIS) retreat.

Description: Instrumental records of the North Atlantic sea surface temperatures (SST) show a significant 60–80 year cycle, referred to as the Atlantic Multidecadal Oscillation (AMO). During AMO warm (cold) phases, SST over the entire North Atlantic Ocean is dominated by basin-wide positive (negative) anomalies. We analyzed SST variations in the North Atlantic Ocean for the last 10 ka. The long-term and centennial variations of Holocene SST in the North Atlantic demonstrate a basin-wide mode that clearly resembles the AMO signal recorded during the recent instrumental period. The long-term changes of Holocene SST were controlled by the solar insolation related to the orbital variations, and the centennial variations were closely coupled with the intensity of the thermohaline circulation. The spatial extent in the Atlantic realm of temperature anomalies around two specific time intervals, 8.2 ka and during the medieval warm period, also resemble the observed temperature anomalies associated with the AMO. These results demonstrate that the modern AMO, and centennial and longer time scale SST variations during the Holocene share a similar spatial extent in the North Atlantic, and presumably as well physical processes associated with their existence and their far-field teleconnection effects.

Description: The isotopic composition of carbon, δ13C, in seawater is used in reconstructions of ocean circulation, marine productivity, air-sea gas exchange, and biosphere carbon storage. Here, a synthesis of δ13C measurements taken from foraminifera in marine sediment cores over the last 150 000 years is presented. The dataset comprises previously published and unpublished data from benthic and planktonic records throughout the global ocean. Data are placed on a common δ18O age scale and filtered to remove timescales shorter than 6 kyr. Error estimates account for the resolution and scatter of the original data, and uncertainty in the relationship between δ13C of calcite and of dissolved inorganic carbon (DIC) in seawater. This will assist comparison with δ13C of DIC output from models, which can be further improved using model outputs such as temperature, DIC concentration, and alkalinity to improve estimates of fractionation during calcite formation. High global deep ocean δ13C, indicating isotopically heavy carbon, is obtained during Marine Isotope Stages (MIS) 1, 3, 5a, 5c and 5e, and low δ13C during MIS 2, 4 and 6, which are temperature minima, with larger amplitude variability in the Atlantic Ocean than the Pacific Ocean. This is likely to result from changes in biosphere carbon storage, modulated by changes in ocean circulation, productivity, and air-sea gas exchange. The North Atlantic vertical δ13C gradient is greater during temperature minima than temperature maxima, attributed to changes in the spatial extent of Atlantic source waters. There are insufficient data from shallower than 2500 m to obtain a coherent pattern in other ocean basins. The data synthesis indicates that basin-scale δ13C during the last interglacial (MIS 5e) is not clearly distinguishable from the Holocene (MIS 1) or from MIS 5a and 5c, despite significant differences in ice volume and atmospheric CO2 concentration during these intervals. Similarly, MIS 6 is only distinguishable from MIS 2 or 4 due to globally lower δ13C values both in benthic and planktonic data. This result is obtained despite individual records showing differences between these intervals, indicating that care must be used in interpreting large scale signals from a small number of records.

Description: The Miocene (~24 to ~5 million years ago) was a period of relative global warmth (e.g. Zachos et al. 2001) characterised by the glaciation of Antarctica only. Paradoxically, the majority of available proxy data suggest that during the Miocene pCO2 was similar, or even lower, than the pre-industrial levels (280 ppmv; Pagani et al., 1999; Pearson and Palmer, 2000; Kürschner et al., 1996, 2008) and at times probably crossed the modelled threshold value required for sustained glaciation in the Northern Hemisphere (DeConto et al., 2008). Records of ice rafted debris and the oxygen isotope composition of benthic foraminifera suggest that at several times over the last 25 million years substantial amounts of continental ice did build up in the Northern Hemisphere but none of these led to sustained glaciation. In this contribution we review evidence that suggests that in the Miocene the North American Cordillera was, at least in parts, considerably lower than today. We present new GCM simulations that imply that Late Miocene uplift of the North American Cordillera would have resulted in significant cooling of Northern North American Continent. Offline ice sheet modelling, driven by these GCM outputs, suggests that with a reduced topography inception of the Cordilleran ice sheet is prohibited, and there is a small, but potentially significant, reduction in the amount of ice grown on Baffin Island. This suggests uplift of the North American Cordillera in the Late Miocene may have played an important role in priming the climate for the intensification of Northern Hemisphere glaciation in the Late Pliocene.

Description: We investigate the relationship between the North Atlantic atmospheric blocking and winter and summer temperature variability as derived from long-term observational and proxy records from southwestern Greenland. It is shown that during boreal winter warm (cold) conditions in southwestern Greenland are related with high (low) blocking activity in the Greenland-Scandinavian region. An index for the North Atlantic blocking is significantly correlated with an oxygen isotope record from Greenland ice cores suggesting a possible reconstruction of blocking variability in this region during past millennium. During summer, high (low) blocking activity in the Euro-Atlantic region is associated with cold (warm) conditions in southwestern Greenland. We conclude that historical temperature records as well as proxy data from Greenland can be used to obtain information related to multidecadal variation of summer blocking during past periods.

Description: The Paleocene-Eocene Thermal Maximum (PETM) occurred approximately 55 million years ago, and is one of the most dramatic abrupt global warming events in the geological record. This warming was triggered by the sudden release of thousands of gigatons of carbon into the atmosphere and is widely perceived to be the best analogue for current anthropogenic climate change. Yet, the mechanism of recovery from this event remains controversial. A massive increase in the intensity of the marine biological pump (''productivity feedback'') has been suggested to cause a drawdown of atmospheric CO2 and subsequent carbon sequestration in the ocean. A re-evaluation of the ''productivity feedback hypothesis'', based on biogenic barium mass accumulation rates (Ba-MARs) for a site in the Southern Ocean, finds that any increase in export production lagged the initial carbon release by at least ~70 000 years. This implies that export production did not rapidly remove excess carbon from the atmosphere, and renders the most likely mechanism for carbon removal to be silicate weathering, at much slower rates than previously assumed.

Description: The impact of vegetation-atmosphere and ocean-atmosphere interactions on the mid- to late Holocene climate change as well as their synergy is studied for different regions in Central and Eastern Asia (60–140° E, 0–55° N), giving consideration to the large climatic and topographical heterogeneity in that area. With main focus on the Asian monsoon, we concentrate on both, temperature and precipitation changes. For our purpose, we analyze a set of coupled numerical experiments, performed with the Earth system model ECHAM5/JSBACH-MPIOM under present-day and mid-Holocene (6 k) orbital configurations (Otto et al., 2009). Like expected, the temperature change caused by the insolation forcing reveals an enhanced seasonal cycle, with a pronounced warming in summer (0.7 K) and autumn (1 K) and a cooling in the other seasons (spring: −0.8 K; winter −0.5 K). Most of this change can be attributed to the direct response of the atmosphere, but the ocean, whose reaction has a lagged seasonal cycle (warming in autumn and winter, cooling in the other seasons), strongly modifies the signal. The simulated contribution of dynamic vegetation is small and most effective in winter, where it slightly warms the near-surface atmosphere (≈0.05 K). Concerning the precipitation, the most remarkable change is the postponement and enhancement of the Asian monsoon (0.27 mm/d in summer, 0.23 mm/d in autumn), mainly related to the direct atmospheric response. On regional average, the ocean (ca. 0.05 mm/d) amplifies the direct effect, but tends to weaken the East Asian summer monsoon and strongly increases the Indian summer monsoon rainfall rate (0.68 mm/d). The influence of dynamic vegetation and synergy effects on precipitation is comparatively small.